1. Field of the Invention
The disclosed embodiments of the present invention relate to calibration methods and associated apparatuses for a communication system, and more particularly, to calibration methods and associated apparatuses for a wireless transceiver based on a Quadrature Amplitude Modulation (QAM).
2. Description of the Prior Art
Transmission rates for communication systems have become higher as the communication systems develop. As a signal modulated by a more complicated modulation technique contains more information than a signal modulated by a less complicated modulation technique, designers can raise the transmission rate by adopting high level complicated process, such as 64-Quadrature Amplitude Modulation (64-QAM), or even 256-QAM. In order to optimize transmission of a high level Quadrature Amplitude Modulation, an Error Vector Magnitude (EVM) of a communication system has to be raised. One of the most significant factors that affect the EVM is In-phase Quadrature-phase imbalance (IQ imbalance), whose root cause is IQ mismatch of a Radio Frequency (RF) circuit, where even a slight mismatch can reduce overall performance of the communication system. Specifically, the imperfect Quadrature Amplitude Modulation/Demodulation introduced by mismatches leads to a poor Bit Error Rate (BER). Mismatches can be divided into amplitude mismatch and phase mismatch. Once these mismatches exist, an unwanted image signal will be generated at symmetric frequency.
Please refer to FIG. 1, which is a diagram illustrating a single tone signal received by a receiver and an image signal generated by the received signal. The difference between the amplitude of the received signal and the amplitude of the image signal is called Image Ratio (IMR). A serious IQ imbalance will lead to a small IMR, and vice versa.
In order to improve the above issues, a calibration process is usually performed before formal signal transmission and reception in a real circuit. The calibration process is called IQ calibration. A communication system may be affected in many ways under many different circumstances, such as different temperatures, different channels, a different low-noise amplifier (LNA) and a different power amplifier (PA). Thus, when and how to perform calibration for an IQ mismatch has become an important issue in this field.